The instant invention is directed to an air-fuel ratio control apparatus for internal combustion engines; more particularly, to an apparatus which uses feedback to control the air-fuel ratio of an engine intake mixture to a target air-fuel ratio.
Japanese First Publication No. 60-240840, published Nov. 29, 1985, describes a conventional air-fuel ratio control apparatus. This conventional apparatus is provided with an oxygen sensor to detect the oxygen density in the exhaust gas, which is representative of the air-fuel ratio of the engine intake mixture, to set an air-fuel ratio feedback correction coefficient. Proportional-integral control is performed based on a comparison of the signal from the oxygen sensor with a level equivalent to the stoichiometric air-fuel ratio, that is, the target air-fuel ratio.
In a conventional system, during cold starting the response characteristics for oxygen density detection degrade due to the low temperature of the oxygen sensor. Therefore, an excessively large control amount for air-fuel ratio feedback control during cold starting makes it difficult to stabilize the air-fuel ratio around the stoichiometric value that results in the best conversion rate for a three-way emissions-control catalyst, even if the catalyst reaches an active state as early as possible as a result of heating by a heater. In this situation, exhaust emission purification is inefficient.
In another conventional apparatus a heater is attached to the oxygen sensor to heat the sensor element in order to activate the oxygen sensor as early as possible. This heater is provided with power in response to ignition switch operation so that the sensor is heated during the non-revolution state of the engine.
The temperature of the oxygen sensor without a heater generally varies with engine temperature. On the other hand, the temperature of the oxygen sensor with a heater varies with heater operating time as well as with engine temperature, which in turn causes variation in the control characteristics, i.e., the amplitude for air-fuel ratio feedback control.